1. Field of the Invention
The present invention relates generally to a method for measuring thermal differences in infrared emissions from micro devices, and more particularly to a method for measuring thermal differences in infrared emissions from semiconductors to detect and locate defects therein.
2. Background of the Invention
It is known that certain physical phenomena, occurring within micro devices such as semiconductor devices, contribute to degradation of the device's performance under various operating conditions. Such phenomena are known to occur, for example, within insulated-gate field-effect transistors (IGFETS), metal oxide semiconductor field-effect transistors (MOSFETS), and virtually all semiconductor devices containing p-n junctions.
MOSFETS are a type of semiconductor device widely applied in large-scale integration, particularly in implementing large random access high-speed memories for computers. One type of phenomena that may contribute to degradation in the performance of MOSFETS is the emission of hot electrons from the MOSFET's silicon substrate into the gate insulator layer under various bias conditions. See generally P. Cotrell, R. Troutman, and T. Ning, "Hot-Electron Emission in N-Channel IGFET's", IEEE Trans. on Electron Devices, Vol. ED-26, pp. 520-33 (1979). It is believed that the resulting substrate current may, in turn, overload the substrate-bias voltage, causing substrate potential fluctuations or electron injection into the substrate, inducing snap-back breakdown and CMOS latchup. Another type of phenomena causing degradation in performance of MOSFETS is believed to be caused by electron trapping in the oxide. Id.
P-n junctions are widely used in various types of semiconductor devices. P-n junctions are formed by placing a p-type semiconductor material adjacent to an n-type semiconductor material, and have the property of blocking the flow of current in one direction while allowing it to pass in the other direction. It is known that certain physical phenomena contribute to breakdown across a p-n junction upon the application of reverse bias. See generally A. Chynoweth and K. McKay, "Photon Emission from Avalanche Breakdown in Silicon", Phys. Rev., Vol. 102, pp. 369-76 (1956). In many cases, such breakdown is undesirable.
The occurrence of these and other undesirable physical phenomena, within semiconductor devices, are known to be accompanied by the emission of electromagnetic radiation. For example, photon emission spectrum characteristics resulting from latchup and hot electrons in n-channel MOSFETS have been measured in the visible spectrum. See, e.g., T. Aoki and A. Yoshii, "Analysis of Latchup-Induced Photoemission", IEDM Technical Digest 89-281, pp. 281-84 (1989). Moreover, emission spectrum characteristics from forward and reversed biased p-n junction diodes have also been measured in the visible spectrum. See A. Chynoweth and K. McKay, "Photon Emission from Avalanche Breakdown in Silicon", Phys. Rev., Vol. 102, pp. 369-76 (1956). These emissions are believed to be generated by Bremsstrahlung radiation, i.e., broad band radiation emission when an energetic electron is decelerated in an electric field.
It is highly desirable to measure the electromagnetic radiation emitted from semiconductors to determine whether the aforementioned and other undesirable phenomena contributing to degradation of performance are occurring or may occur under certain operating conditions. It is also desirable to spatially locate from where, within the micro device, the radiation is emitted. Detections of such radiation emission, by measuring the "thermal signature" of the device, can form the basis for testing failure mechanisms in semiconductor devices, and can be used to detect defects in individual semiconductors or locate problems in various manufacturing processes. Moreover, such measurement can be utilized to predict failures in semiconductors and to improve overall design.
Most failures of semiconductor devices will be accompanied by abnormal thermal signatures, e.g., some devices might become very hot while some might never turn on when powered up. The change in thermal signature due to the failure mechanism, however, may be difficult to view, due to operating temperature, small thermal gradient, or small device size.
In the past, measurements of such electromagnetic radiation emissions from semiconductors have been performed in the visible spectrum. This basic technology has now evolved into photoemission microscopy and is being utilized in the private sector in testing failure mechanisms in semiconductor devices.
It is desirable, however, to measure radiation emissions in other wavelengths of the electromagnetic spectrum. This will provide a more complete spectral analysis of the emitted radiation and thermal signatures in semiconductor devices that result from failure mechanisms, thus providing additional test data. Moreover, the applicant believes that certain undesirable phenomena produce infrared radiation, but do not generate radiation in the visible spectrum. Therefore, detection of such phenomena cannot be accomplished using measurements limited solely to the visible spectrum.
The measurement of infrared radiation emission from micro devices, however, is difficult because substantial infrared background radiation is emitted from all objects at or near room temperature. Moreover, some failed devices with abnormal thermal signatures generate large amounts of infrared background radiation. The applicant believes that no practical method currently exists to measure thermal differences in infrared emissions from micro devices with adequate sensitivity, resolution, frame speed, and measurement accuracy.